The ancient structure built as a tomb for a Roman emperor is the **Mausoleum of Hadrian**, commonly known as the **Castel Sant'Angelo**. Constructed in the 2nd century AD, it was commissioned by Emperor Hadrian for himself and his family. Initially built as a monumental tomb, it later served various purposes, including a fortress and papal residence. The structure is notable for its cylindrical design and grand terrace topped with a statue of an angel, which added to its mystique. Today, Castel Sant'Angelo is a prominent historical site and museum that attracts numerous visitors each year.